The effects of oxidizing conditions on vulcanizates obtained from polymers have long been a problem, particularly in applications where the vulcanizates are exposed to elevated temperatures for extended periods of time. A variety of approaches have been developed in the art in an attempt to solve this problem.
It is known that, in compositions comprising polymers based on a monomer which results in a polymer backbone having repeating units including at least one carbon-hydrogen bond (i.e., repeating units have a secondary or tertiary carbon), thermo-oxidative attack initiated by a radical mechanism is very relevant in the deterioration of the useful properties of such compositions during oxidative aging. See, for example:                1. S. Bhattacharjee, A. K. Bhowmick and B. N. Avasthi: “Degradation of Hydrogenated Nitrile Rubber”; Polymer Degradation and Stability, 31, 71–87 (1991); and        2. K. C. Smith and B. S. Tripathy: “HNBR and Long Term Serviceability in Modern Automotive Lubricants”; Rubber World, 217 (5), 28–45 (1998).        
During the oxidative degradation process located at such carbon-hydrogen bonds among other substances hydroperoxide, alcohol, keto, aldehyde and carboxylic acid functionalities are introduced into the main polymer chain (also referred to as the “polymer backbone”). This often results in polymer chain scission or crosslinking reactions which lead to changes and deterioration of the useful properties of the composition such as tensile strength, hardness, static and dynamic stiffness, elongation at break, compression set etc.
Thermo-oxidative reactions as described above are autocatalytic chain reactions, where reactive radicals are regenerated within the reaction cascade. It is known in the art to add substances (often called antioxidants) to polymer compositions to facilitate destruction of radicals or reactive intermediates produced during the polymer oxidation process (such as hydroperoxides) thereby improving the oxidative heat aging resistance of the compositions.
Non-limiting examples of useful antioxidants may be selected from the group including hindered phenols, p-phenylene diamine derivatives, quinoline derivatives and mixtures thereof. Phosphites, dithiophosphates, dithiocarbamates and mercaptoimidazole derivatives are also commonly employed as antioxidants. These substances often either donate hydrogen atoms to other radicals and, during the polymer oxidation process, they:                (i) are converted into unreactive radicals themselves;        (ii) block certain reactions which lead to the production of free radicals (e.g., heavy metal trapping); and/or        (iii) favor reactions of reactive intermediates leading to the production of non-radical reaction products (e.g., hydroperoxide decomposer).        
In many cases, to achieve their desired properties, rubber compositions are cured with a crosslinking system conventionally selected from the group comprising sulfur, sulfur donor compounds and/or a peroxide system. It is known in the art that interference of antioxidants with cure systems often presents a major problem. Reaction of antioxidants with cure systems may lead to significant deterioration of the desired state of cure of the composition. Complete or partial depletion of the antioxidant in the composition during cure is likely to occur when the cure system generates radicals during vulcanization.